This invention relates to the field of radio frequency (RF) shielding, and more particularly to an electrically conductive gasket for use between adjacent conductive sections of a shielded enclosure.
A common problem associated with shielded enclosures is the loss of shielding effectiveness at the gap between the door and the door frame. Several approaches to providing a continuous electrically conductive medium between the door and the door frame have been employed in the past.
One approach involves the use of conductive finger stock on the door or door frame of the shielded enclosure. The finger stock is positioned to contact with a knife-edge extension on the opposite surface thereby establishing electrical connection between the two surfaces.
Another common approach involves the use of a wire mesh gasket which is mounted to the doors and hatches of a shielded structure. This comprises a solid polymer core surrounded by several concentric rings of conductive wire mesh. The gasket is made to fit within a channel around the perimeter of the door. As the door is closed, the gasket is compressed between the door channel and the frame of the shelter, thereby providing the necessary contact pressure between the gasket and the mating surfaces to establish electrical conductivity. The core should maintain a nearly constant pressure between the mating surfaces regardless of how many times it has been deformed in this manner.
With repeated use, however, the gasket core suffers permanent deformation, or "set", and fails to provide a constant pressure between the mating surfaces. In addition, gasket set frustrates the "cleaning action" experienced when a gasket is compressed onto a small bead, such as that found on a mating surface. Ordinarily, the sides of the bead rub against the gasket surface and scrape off oxides and other impurities which might impair electrical contact.
Experience has shown that the shielding effectiveness of mesh gaskets with polymer cores degrades rapidly after only a few months, after which time the permanent deformation adversely affects the gasket's ability to provide adequate shielding.
A discussion of the disadvantages inherent in these approaches can also be found in U.S. Pat. No. 4,177,353 (1979) to McCormack. McCormack teaches an inflatable tube with a sleeve or coating of conducting material for use as an RF gasket. The tube is fixedly mounted about the entire periphery of the door frame and is inflated with compressed air or some other compressed gas when the door is closed so as to provide electrical contact between the door and the shielded structure. The gasket must be deflated to either open or close the door.
A similar solution is proposed by Van Dyk, U.S. Pat. No. 4,371,175 (1983). A flattened inflatable elastomeric tube is disposed along the periphery of a conductive section of the shielded enclosure and provided with a loosely fitting conductive sheath. When the door to the enclosure is closed, the tube is inflated through introduction of compressed fluid such as air, thereby urging the shielding medium into conductive contact with adjacent sections of the enclosure.
The inflatable gaskets proposed by McCormack and Van Dyk must be deflated to either open or close the door to the enclosure. For this reason large volumes of compressed air or gas ar required for operation. This in turn necessitates the use of expensive equipment such as air compressors, and the constant availability of power. Air or gas tanks can be used, but in many applications these exceed size and weight limitations.
In addition, where conductive outer shells are used in conjunction with an elastomeric tubular core, the mesh must be large enough to accommodate the maximum expanded size of the hollow gasket core. Thus, when the gasket is deflated, the outer shell does not reduce in size and will become "baggy" in the gasket channel. This can cause two problems: first, it allows the gasket to pinch, preventing uniform surface conductivity, and second, it prohibits the natural cleansing action which occurs when a snug outer shell is pressed against the bead on the door frame.